1. Field of the Invention
The present invention relates to a laser printer, and more particularly, to a high developing voltage supply apparatus for developer fixed non-contact-type color laser printers.
2. Description of the Prior Art
A laser printer is a device designed to reproduce images on a printing medium and has advantages of low noise and fast speed. The laser printer generally performs printing jobs on the printing medium while sequentially carrying out operations such as electric charge—exposure to light—development—transfer—cleaning—electric discharge, for which the laser printer is provided with diverse mechanisms and electric circuits.
FIG. 1 is a view schematically showing a structure of a conventional developer-moving color laser printer. The developer-moving color laser printer has an organic photo conductor(OPC) drum 10, an electric charge unit 20, a laser scanning unit(LSU) 30, a development unit 40, a transfer unit 50, and a cleaning unit 60. In addition to the above, the laser printer includes a fixing unit, an electric discharge unit, an electric power supply unit, a high-voltage supply unit, an engine control unit, which are not shown.
The electric charge unit 20 electrically discharges a high-voltage to a surface of the OPC drum 10 to electrically charge the surface of the OPC drum 10 with positive(+) and negative(−) charges.
The LSU 30 scans the surface of the OPC drum 10 with a laser beam, based on an image to be printed to form an electrostatic latent image on the charged surface of the OPC drum 10.
The development unit 40 has development rollers, each associated with a color, and supplies toner on the surface of the OPC drum 10 on which the electrostatic latent image is formed.
The transfer unit 50 has a transfer belt 50a and a convey unit 50b moving the belt 50a, so as to transfer to the printing medium a toner image developed on the OPC drum 10.
Further, the cleaning unit 60 is closely installed nearby the surfaces of the OPC drum 10 and the transfer belt 50a, respectively, to remove the toner remaining on surfaces of the OPC drum 10 and the transfer unit 50.
In the laser printer as described above, the electric charge unit 20 electrically charges the surface of the OPC drum 10, and the LSU 30 scans the electrically charged surface of the OPC drum 10 with the laser beam based on the image to be printed so as to form the electrostatic latent image on the surface of the OPC drum 10. Further, the toner is applied to the surface of the OPC drum 10 through individual development rollers K, M, C, and Y by color, of the development unit 40 Accordingly, the toner image is formed on the surface of the OPC drum 10. At this time, the individual development rollers K, M, C, and Y are initially spaced at a predetermined distance from the OPC drum 10, and only the development roller K, M, C, or Y for the color to be used for printing moves toward a direction of the OPC drum 10 by a driving development unit in the development unit 40. Further, the development roller K, M, C, or Y keeps a predetermined gap from the OPC drum 10 and performs the development by an applied voltage. Thereafter, the toner image formed on the surface of the OPC drum 10 is transferred to the surface of the transfer belt 50a, and the toner image transferred to the transfer belt 50a is transferred again to the printing medium being fed. The image-transferred printing medium is fixed onto the printing medium by the fixing unit (not shown) and externally discharged from the printer. At this time, the toner remaining on the transfer belt 50a and the OPC drum 10 is removed by a blade 60a of the cleaning unit 60 contacted on the surfaces of the belt 50a and the drum 10 so that the belt 50a and the drum 10 can be prepared for a next image. Once removing of the toner is completed, the electric charges existing on the surface of the OPC drum 10 are removed in order for the drum 10 to return to the initial state.
In the printing process of the above developer-moving color laser printer, the development of the toner image is achieved by sequentially moving the individual development rollers Y, M, C, and K, that is, yellow(Y), magenta(M), cyan(C), and Klack(B) toners to the surface of the OPC drum 10 based on color characteristics of image data.
Further, in an operation in which a toner image formed on the surface of the OPC drum 10 is transferred to the printing medium, the toner image formed on the surface of the OPC drum 10 is superimposed and transferred onto the transfer belt 50a, and the toner image superimposed onto the transfer belt 50a is transferred on the printing medium at one time. Accordingly, in order to print one color image, the OPC drum 10 and the transfer belt 50a has four times a revolution speed respectively.
However, the laser printer using the developer-moving non-contact developing method as described above has a problem in that, as the development roller move to the OPC drum 10, the revolution speed of the OPC drum 10 decreases due to a shock of the OPC drum 10 and, as the development rollers are released from the OPC drum 10, the revolution speed of the drum 10 increases so that bending occurs on the printing medium due to the changes to the revolution speed of the OPC drum 10.
In order to solve the problem of deterioration of the image quality due to the changes of the revolution speed of the OPC drum 10 in the developer-moving non-contact laser printer as described above, a developer-fixed non-contact laser printer has been developed to carry out the developments while keeping a certain gap from the OPC drum 10 without moving the development rollers Y, M, C, and K.
The developer-fixed laser printer performs developments without moving the developers in the state that all of the development rollers Y, M, C, and K maintain a predetermined gap from the OPC drum 10, respectively. Further, a high developing voltage supplied to the respective developers is controlled in use of mechanical contacts.
FIG. 2 is a view showing an electrical circuit for a high developing voltage supply apparatus of the conventional developer-fixed non-contact color laser printer. The high developing voltage supply apparatus has a high developing voltage power supply (HVPS) 42 for supplying the high voltage in order for the toner to move to the OPC drum 10 from the fixed development rollers Y, M, C, and K by color respectively, a high voltage driver 44 for selectively outputting a high voltage color drive signal to apply a high developing voltage supplied from the high developing voltage supply 42 to respective color development rollers Y, C, M, and K based on the images to be printed, and solenoid power switches SY, SC, SM, and SK switching the high developing voltages supplied from the high developing voltage supply 42 to the color development rollers Y, C, M, and K in response to a color driving signal outputted from the high voltage driver 44. In FIG. 2, a reference numerals ‘CA*’ denote a capacitance due to a gap between the individual color development rollers Y, M, C, and K and the OPC drum 10, and ‘CP’ capacitance of the OPC drum 10 of a dielectric substance.
In order to perform the developments in the non-contact-type color laser printer, the high voltage is supplied by adding an AC voltage to a DC voltage because an electric field decreases in an inverse proportion to the gap of the OPC drum 10 and the development rollers Y, M, C, and K causes toner to move from development roller Y, M, C, or K to the OPC drum 10. That is, when a high AC voltage VAC is applied to the development roller, the toner moves to the OPC drum 10 if a developing vector is negative when a minus toner is used. The toner in the developer does not move if the developing vector is positive, because a direction in which the toner moves is determined based on the developing vector (a DC component of a developing bias—a potential of the surface of the OPC drum 10).
FIG. 3 and FIG. 4 are views for showing a voltage waveform and an electric field waveform, respectively, when the AC voltage is superimposed to the DC voltage. Because the developing vector is positive, the toner is not applied to the OPC drum 10, but stays on the surface of the development roller. Further, if the developing vector becomes negative, the component of a developing electric field toward the OPC drum 10 becomes relatively larger so that the toner is applied to the OPC drum 10.
However, in the developer-fixed laser printer as above, when a developing operation is performed, a development roller to which a high developing voltage is not applied has charges induced from the charges of the surface of the development roller and an ambient charges because the metal shaft of the developer roll is installed floated so that the potential of the metal shaft becomes very unstable. Accordingly, if a development has been made on the surface of the OPC drum 10 by a color toner of one of the development rollers and the developing vector becomes positive, as shown in FIG. 5, the toner developed on the surface of the OPC drum 10 by a preceding color developer reversely moves to the surface of the the one of the development rollers, causing a problem of producing cross contamination upon printing. The cross contamination changes the colors of the image, which becomes a cause degrading image quality.